1. Field of the Invention
The present invention relates to an organic electroluminescent device (ELD), and more particularly to a dual plate type organic ELD and a method of fabrication thereof.
2. Discussion of the Related Art
In general, an organic ELD emits light by injecting electrons from a cathode and holes from an anode into a luminescence layer, combining the electrons with the holes, generating excitons, and transitioning the excitons from an excited state to a ground state. Compared to a liquid crystal display (LCD) device, an additional light source is not necessary for the organic ELD to emit light because the transition of the excitons between the two states causes light to be emitted. Accordingly, the size and weight of the organic ELD can be reduced. The organic ELD has other excellent characteristics such as low power consumption, superior brightness, and fast response time. Because of these characteristics, the organic ELD is regarded as a promising display for next-generation consumer electronic applications such as cellular phones, car navigation system (CNS), personal digital assistants (PDA), camcorders, and palmtop computers. Moreover, since fabricating the organic ELD is a simple process with few processing steps, it is much cheaper to produce an organic ELD than an LCD device.
Two different types of organic ELDs exist: passive matrix and active matrix. While both the passive matrix organic ELD and the active matrix organic ELD have a simple structure and are formed by a simple fabricating process, the passive matrix organic ELD requires a relatively high amount of power to operate. In addition, the display size of a passive matrix organic ELD is limited by its structure. Furthermore, as the number of conductive lines increases, the aperture ratio of a passive matrix organic ELD decreases. In contrast, active matrix organic ELDs are highly efficient and can produce a high-quality image for a large display with a relatively low power.
FIG. 1 is an energy band diagram of an organic ELD. As shown in FIG. 1, an anode 1 and a cathode 7 are spaced apart from each other. A luminescence layer 4 is interposed between the anode 1 and the cathode 7. A hole transporting layer 3 is interposed between the anode 1 and the luminescence layer 4, and an electron transporting layer 5 is interposed between the luminescence layer 4 and the cathode 7 to improve luminescence efficiency. The hole injected from the anode 1 and the electron injected from the cathode 7 combine in the luminescence layer 4, and light corresponding to an energy between the hole and the electron is emitted from the excitons. The anode 1 may be made of a transparent conductive material having a high work function and, the cathode 7 may be made of a metallic material having a low work function and stable chemical property.
FIG. 2 is a schematic cross-sectional view of a bottom emission type organic ELD according to a related art. In FIG. 2, an array element 14 including a thin film transistor (TFT) “T” is formed on a first substrate 10. A first electrode 16, an organic electroluminescent (EL) layer 18, and a second electrode 20 are formed over the array element 14. The organic EL layer 18 may separately display red, green, and blue colors in each pixel region. Generally, the divided organic materials are used to emit light of each color for the organic EL layer in each pixel region. An organic ELD is encapsulated by attaching the first substrate 10 to a second substrate 28 with a sealant 26. The second substrate 28 includes a moisture absorbent material 22 to eliminate moisture and oxygen that may penetrate into a capsule of the organic EL layer 18. After etching a portion of the second substrate 28, the etched portion is filled with the moisture absorbent material 22 and the filled moisture absorbent material is fixed by a holding element 25.
FIG. 3 is an equivalent circuit diagram of the organic ELD according to the related art. In FIG. 3, a gate line “GL” crosses a data line “DL”, and a switching element “TS” at a crossing of the gate line “GL” and the data line “DL” is connected to the gate line “GL” and the data line “DL.” A driving element “TD” is electrically connected to the switching element “TS” and an organic electroluminescent diode “DEL.” The switching element “TS” includes a switching source electrode “S4”, a switching gate electrode “S2” and a switching drain electrode “S6”. Also, the driving element “TD” includes a driving source electrode “D4”, a driving gate electrode “D2” and a driving drain electrode “D6”. A storage capacitor “CST” is formed between the driving gate electrode “D2” and the driving drain electrode “D6” of the driving element “TD,” and the organic electroluminescent diode “DEL” is connected to a power line “PL.”
When a scan signal of the gate line “GL” is applied to the switching gate electrode “S2” of the switching element “TS,” an image signal of the data line “DL” is applied to the driving gate electrode “D2” of the driving element “TD” through the switching element “TS.” The current density of the driving element “TD” is modulated by the image signal applied to the driving gate electrode “D2.” As a result, the organic electroluminescent diode “DEL” can display images with gray scale levels. Moreover, because the image signal stored in the storage capacitor “CST” is applied to the driving gate electrode “D2,” the current density flowing into the organic electroluminescent diode “DEL” is uniformly maintained until the next image signal is applied, even when the switching element “TS” is turned off. The switching element “TS” and the driving element “TD” may be a polycrystalline silicon TFT or an amorphous silicon TFT. The process of fabricating an amorphous silicon TFT is simpler than the process for a polycrystalline silicon TFT.
Meanwhile, the organic ELD should have red, green and green organic EL layers in each pixel region to display full color. The organic EL layers are divided by separators having an inverted trapezoid shape to provide a stable coating process for the organic EL layer. In an organic ELD having such separators, the organic EL layers are formed by dropping or injecting a solution type organic EL material in the pixel regions surrounded by the separators.
FIGS. 4A to 4F are schematic cross-sectional views illustrating a fabricating process of an organic ELD including an inkjet-printing step according to the related art.
In FIG. 4A, a substrate 30 includes emission regions “C” and non-emission regions “D” adjacent to the emission regions “C.” First electrodes 32 (an anode) are formed on the substrate 30 in the emission regions “C” by depositing a transparent conductive material. In addition, a buffer layer 34 is formed on the substrate 30 in the non-emission regions “D” and overlaps an edge portion of the first electrodes 32. The buffer layer 34 may be made of silicon nitride (SiNx) or silicon oxide (SiO2). In addition, thin film transistors (TFTs) “T” are formed between the substrate 30 and the first electrodes 32, and are connected to the first electrodes 32 in each of the pixel regions “P.” Although not shown in FIG. 4A, the TFT “T” includes a switching TFT and a driving TFT, and the first electrode 34 are connected to a driving drain electrode of the driving TFT.
In FIGS. 4B and 4C, an organic material layer 40 is coated over an entire surface of the substrate 30 having the first electrodes 32 and the buffer layer. Then, a photo-resist layer 45 is coated on an entire surface of the organic material layer 40, and a mask 50 having light-transmitting portions “m1” and light-shielding portions “m2” is disposed over the substrate 30. The photo-resist layer 45 is patterned by a photolithography process, which includes exposure and development steps. Photo-resists are classified into a positive type and a negative type. When the photo-resist layer 45 is a positive type, then the light-transmitting portions “m1” exposed to light are removed during the development step. On the other hand, when the photo-resist layer 45 is a negative type, then light-shielding portions “m2” are removed during the development step. In FIGS. 4B and 4C, because the photo-resist layer 45 is a negative type photo-resist, the portions of the photo-resist layer 45 corresponding to the light-shielding portions “m2” are removed. Thus, the photo-resist layer 45 corresponding to the light-transmitting portions “m1” remains as photo-resist patterns 46, as shown in FIG. 4D.
In FIG. 4D, a plurality of separators 41 are formed by etching the organic material layer 40 (of FIG. 4C) using the photo-resist patterns 46 as a mask. During the etching step, the exposed organic material layer 40 (of FIG. 4C) between the photo-resist patterns 46 is removed by an etchant. The separators 41 prevent color mixing, because the separators 41 surround each pixel region “P.” Although not shown in FIG. 4D, the photo-resist patterns 46 are stripped off after forming the separators 41.
In FIG. 4E, red, green and blue organic EL layers 42a, 42b and 42c are formed on the substrate 30 having the separators 41 by inkjet-printing red, green and blue organic EL materials. The red, green and blue organic EL layers 42a, 42b and 42c may be formed by dispensing each color organic EL material through an inkjet nozzle (not shown) in the pixel regions “P” surrounded by the separators 41. Because the separators 41 are located at boundaries of the pixel regions “P” and have a predetermined height, color mixing of the red, green and blue organic EL layers 42a, 42b and 42c can be prevented at the boundaries of the pixel regions “P.”
In FIG. 4F, a second electrode 44 is formed over the substrate 30 having the red, green and blue organic EL layers 42a, 42b and 42c in the pixel regions “P.” When the second electrode 44 functions as a cathode, the second electrode 44 may be a metallic material having a low work function and a good reflectivity.
However, in order to form the separators using an organic material, a photolithography process is required, which includes the steps of coating, exposure and development. Therefore, production cost increases. In addition, because the height of the separators formed of an organic material is relatively high, a metal layer that will be formed later on the organic electroluminescent layer may be undesirably cut during the deposition of the metal layer, leading to a manufacturing defect. Moreover, the organic material dispensed in the pixel regions by an inkjet method is not uniform. In particular, the thickness of the organic material near the separators are thicker than the thickness of the organic material in other areas, as illustrated in FIG. 4F. This is called a pinning phenomenon which is caused by the surface tension between the organic material and the separators.